I agree with previous sentiments about the value of this at $300. You can go out and buy something like uATX or ITX board and a small case for similar money. There are drop in linux distros that will act as NAS boxes and provide web interfaces, rolling your own isn't that hard. Or pay a bit more and get something like the HP mediasmart home server for only $100 more. Lots more configurability.
However I purchased a Maxtor Shared Storage II single drive unit for about 100 a couple years back (with built in 320GB, awesome deal). I like the simplistic nature of it, low power use, flawless drive standby, and it runs a USB print server reliably so I can print from any computer. I didn't want RAID since I "rsync" the data over the network for redundancy when my main computer is on (good thing too, I had to have the hard drive replaced under warranty). I didn't need gigE though I may look to move to it at some point.
So for the right price, these things can be nice, but are limiting. For instance my brother has a squeezebox and while the NAS had a itunes server, it cant run the slimserver. That would be no problem on a HTPC type build with linux or WHS. Other NAS have slimserver capability but I still posit that they are less flexible than a full buildup. I can live with that, but not for $300+ dollars.